1. Field of the Invention
The field of Microelectromechanical Systems (MEMS) uses a variety of fabrication technologies, such as surface micromachining developed for the integrated circuit industry, to create highly miniaturized mechanical devices (usually 1 .mu.m to 1 mm in size) on a microelectronic chip. This invention is in the field of microelectromechanical systems (MEMS), and in particular relates to post fabrication metallization techniques used to create interconnects for MEMS devices.
2. Description of the Prior Art
The majority of microelectromechanical systems (MEMS) in use today are fabricated in a variety of surface micromachining processes. Surface micromachined devices are formed by the alternate deposition of structural layers to form the device and sacrificial spacer layers to provide mechanical and electrical isolation. Polycrystalline silicon (polysilicon) is the most commonly used structural material and silicon dioxide (oxide) glass is the most commonly used sacrificial material. These layers, formed above a silicon substrate isolated with a layer of silicon nitride, are patterned (using the same advanced photolithography technology employed by the microelectronics industry) to form intricate structures such as motors, gears, mirrors, and various sensors. Cuts made through the oxide layers are used to anchor the upper structural levels to the substrate or to the underlying mechanical structures. At the end of the process, the sacrificial layers are removed using various techniques, such as a hydrofluoric acid release etch, which frees the device to move relative to the substrate. (M. A. Michalicek, J. H. Comtois, and H. K. Schriner, "Design and fabrication of optical MEMS using a four-level, planarized, surface-micromachined polysilicon5 process," Proc. SPIE, Vol. 3276, pp. 48-55, 1998.)
The complexity of the micromachines that can be manufactured in a given process is a function of the number of independent layers of structural material the technology provides. A single independent level of structural material limits designers to simple sensors. Geared mechanisms require two releasable structural layers, a Poly-1 layer to form the gears and a Poly-2 layer to form the locking hub above the ground layer (Poly-0). Motorized geared mechanisms require a minimum of three independent levels. Far more complex mechanisms and systems require even more structural layers.
Surface micromachining fabrication of electronics and MEMS is well developed and widely used both privately and commercially. Countless companies, universities, and government agencies have fabricated micromechanical devices for the last 10 years or more. However, one problem still plaguing designers is the limited number of wiring bonds that can be made per chip. Quite frequently there are more devices on the chip than there is room to place sufficient bond pads around the edge of the test chip. As a result, not all chips are made as functional as possible because many devices had to be wired together or not wired at all and therefore could not be electrically actuated and tested. To overcome this limit, a novel post-process metallization wiring interconnect was developed in which selected micromechanical devices can be wired to a "bus" of probe pads by masking and depositing a conductive material after the chip is fabricated.